The invention relates to a method in the surface sizing of a paper or board web, in which method surface size, such as a starch solution, is applied by means of an applicator device to at least one side of the web to be surface sized, and in which surface sizing is performed per one side of said web in one or more stages. The invention also relates to an apparatus in the surface sizing of a paper or board web, which apparatus has been arranged to apply surface size, such as a starch solution, by means of an applicator device in one or more stages to at least one side of the web to be surface sized.
In the manufacture of writing papers and many packing boards, surface sizing is used in which a dilute starch solution is applied to a paper or board web.
However, the object of surface sizing is not only to form a special layer onto the surface of the web that is being treated, but the size, such as a dilute starch solution, must penetrate to a certain extent into the web to bind the fibers in the surface layer of the web into a homogeneous layer. Several different methods and apparatuses are known in the prior art for spreading and applying size to the paper or board web. These prior-art methods and apparatuses include, among other things, pond coating, in which the web to be coated is passed through a size press nip formed by rolls, and a size pond is arranged on the incoming side of the nip in the closing gap defined by the rolls, the web to be coated being passed through said size pond. Another known coating and surface sizing method is film transfer coating, in which coating material or size is spread in a suitable fashion, for example, using a blade or a rod to form a film on the surface of a film press roll, from which said film is then transferred in a nip between the film press rolls from the surface of the roll to the web which runs through the nip and which is being coated. The coating methods known in the art further include different blade coating arrangements, in which, for example, the web to be coated or surface sized is passed over a backing roll and coating material or equivalent surface size is spread and smoothed onto the web by means of a blade coater. The spreading of size onto the web can also be arranged to take place, for example, by means of a jet applicator in accordance with Finnish Patent No. 108993, a curtain coating device or a spray applicator, which represent the latest surface sizing techniques. By means of the spray coating method, a very even coating material or surface size layer of desired thickness can be provided on the surface of the web.
Although the above-mentioned and previously known surface sizing methods are arrangements that are in use and operative, certain problems or different restrictions are associated with each of them. Thus, as known, in surface sizing the size material must be made to penetrate in a desired manner into the inner layers of the web. In particular, when surface sizing thicker packing boards, it is the penetration of starch that becomes a critical factor. In pond sizing, penetration can be made fairly good, but a significant drawback in this method is the speed limitation of the method. The speed cannot be raised to a very high level, among other things, because of the splashing of size material. In film sizing it is generally possible to use higher speeds, but in this surface sizing method, in particular with thick board grades, insufficient penetration of size material into the board web being treated becomes a problem. This is the case in spite of the fact that attempts are made to bring penetration to a desired level by means of the linear load prevailing in the size press nip. Spray and jet applicators are fairly well suited for use in the coating and surface sizing of thin paper and board grades and, in addition, fairly considerable speeds can be achieved by these coating methods. As such, however, these coating methods are not suitable for the manufacture of paper and board grades that require starch penetration because contact-free coating is used as the coating method.
To improve the penetration of surface size, it has been proposed previously that surface sizing be performed in two or more successive stages, for example, such that part of the total amount of size is first added to the web before the drying of the web is started by heating, and at least part of the total amount of size is added to the surface of the web after the drying has been started. One such method is described in the publication WO 03/004769. In the method described in this publication, the entire amount of size is thus not added in a single stage but in several stages. To improve penetration, the publication WO 03/004770 further proposes that the web be pressed in a nip between rolls after the addition of size.